This invention relates to a novel method for producing a glossy, marbleized, tough, optionally smooth surface on a lignocellulosic panel such as wood, plywood, fiberboard or particleboard.
Untreated surfaces of conventional plywood, fiberboard or particleboard, as indeed those of solid wood itself, are known to have low resistance to abrasion and to be prone to dimensional changes on contact with moisture. Such dimensional changes involve local areas of relative swelling and shrinking, resulting in visual deficiencies which are known as face-checking, grain-raising and the like. The surfaces of particleboards and so-called fiberboards have low decorative value as is also the case with solid wood and plywood when these are made unsightly by knots and knotholes.
Various methods have been used conventionally to provide coatings for panels with the purpose of overcoming some of the aforementioned surface deficiencies. Various types of paints, varnishes, lacquers and the like have been used. In particular among these, amino-modified alkyd resin compositions with solvent and optionally with pigment, have been applied by spraying or roller coating and then oven-treating to evaporate the solvent and cure the polymer to provide a baked enamel finish. Such finishes, however, are usually poor in wearability and in resistance to solvents and chemical stains. The evolution of organic solvents during application causes an air pollution problem. Furthermore, unless the panel surface is first smoothed by a sanding procedure, the surface roughness is "telegraphed" to the surface of the baked enamel finish and the enamel surface has a roughness which is a replica of the roughness on the original panel.
When the conventional oils, paints or varnishes are used, the improvement in appearance is only temporary. Even with up to five successive coats, the barrier to moisture is usually not sufficient to prevent eventual distortion due to swelling and shrinking of the panel.
A type of surfacing known as "overlay" is accomplished by first treating a fibrous web such as paper or cotton fabric with a synthetic polymer of either the thermosetting or thermoplastic type and then laminating the treated web to the substrate panel. Alternatively, thermoplastic films have been used as overlays. When paper treated, exemplarily, with melamine formaldehyde and/or phenol formaldehyde thermosetting resin is used as an overlay, excellent wearability and resistance to stains and solvent are obtained. However, for best results in laminating thermosetting resins, high pressures, up to as high as 1,200 psi, are required, which has the side effect of tending to break down the internal bonds of the panel substrate. Furthermore, to obtain a uniform surface, the platen should be cooled to about 50.degree. C under pressure after each heating cycle.
A fibrous type overlay was disclosed by Bryant (Forest Products Journal 18 (5) :51-58 [1968]) using mechanical wood fibers that were pre-coated with a thermosetting resin. The resin-bound fibers were applied as a mat which had a short duration of pre-curing stability. The direct application of dried phenolic resin powder on the board surface together with wood fibers followed by hot pressing has also been tried; the aforesaid high curing pressure and repeated cooling cycles were still necessary.
Vinyl modified lignocellulosic paper as a top liner for cylinder board is disclosed in Examples 9 of U.S. Pat. Nos. 3,194,727 and 3,395,070. Details of the structure, however, are not set forth.
Corresponding use of thermoplastics in resin paper overlays also has had several disadvantages. The requirement of high alpha-cellulose pulp or paper makes such procedure expensive. The tackiness of polyester-treated paper sheet requires the extra complication of using cellophane separators in handling. Paper overlays made by saturating with acrylic latexes have had deficiencies in heat stability and resistance to organic solvents even when the ratio of acrylic polymer to cellulose was limited to not more than 30%. In order to obtain a smooth, glossy surface, the platen must not be removed after hot pressing until it has been cooled to a temperature below the glass transition temperature of the impregnated thermoplastic polymer.
To the best knowledge of the Applicants, no practicable method has been proposed whereby a particulate composition comprising cellulosic or wood particles merely admixed with thermoplastic polymer particles can be used successfully to prepare a smooth, glossy, tough surface on a wood-based panel and in particular without the necessity of repeated cyclic cooling of the platen to a temperature below the glass transition temperature of the thermoplastic polymer.